Encapsulated fuse for printed circuits



United States Patent 3,213,414 Patented Nov. 16, 1965 3,218,414 ENCAPSULATED FUSE FOR PRINTED CIRCUITS Kenneth W. Swain, Hampton Falls, and Frederick J. Kozacka, South Hampton, N.H., assignors to The Chase-Shawmut Company, Newburyport, Mass.

Filed Dec. 15, 1961, Ser. No. 159,535 Claims. (Cl. 200120) This invention refers to electric fuses, and more particularly to miniature fuses for application in electronic circuitry.

It is common practice to employ in electronic circuitry so-called glass fuses which are small fuses having a tubular casing of glass housing a thin fusible wire, the easing of glass being filled with a pulverulent arc-quenching filler if the interrupting capacity of the fuse is to exceed predetermined limits.

Glass casings break relatively easily, this being in many instances a serious limitation of so-called glass fuses. In case of miniature fuses the use of pulverulent substances as arc-quenching means becomes a serious manufacturing problem.

It is, therefore, an object of this invention to provide miniature fuses which lend themselves for application in electronic circuitry, and which are not subject to the two aforementioned limitations, or drawbacks.

Another object of this invention is to provide improved fuses having terminals adapted to be soldered into socalled printed circuits.

Still another object of the invention is to provide fuses which do not comprise any casing and pulverulent arcquenching filler, i.e., wherein the casing and the pulverulent filler normally encountered in fuses are entirely dispensed with, and wherein one single means performs the dual function of enclosing the constituent parts of the fuse and of operating as a de-ionizing or arc-energyabsorbing means.

Further objects and advantages of the invention will become apparent as this description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to, and forming part of, this specification.

For a better understanding of the invention reference may be had to the accompanying drawing wherein:

FIG. 1 shows-substantially enlarged-a first embodiment of the invention taken along 11 of FIG. 2;

FIG. 2 is a section along 22 of FIG. 1;

FIG. 3 shows-substantially enlarged-a second embodiment of the invention taken along 3-3 of FIG. 4; and

FIG. 4 is a section along 4-4 of FIG. 3.

Referring now to the drawings, and more particularly FIGS. 1 and 2 thereof, reference character 1 has been applied to indicate a cylindrical supporting body, which body is an integral unit of copper-clad insulating material. Supporting body 1 is a stamping formed from a portion of a larger sheet of metal-clad laminate. Body 1 includes outer layers In of metal, preferably copper, sandwiching an inner layer 11) of an organic insulating material evolving gas when subjected to the heat of an electric arc. Bore 2 extends transversely through outer layers 1a and inner layer 1b. The diameter of bore 2 is very small in comparison to the diameter of stamping or cylindrical supporting body 1. A very thin silver wire 3 is threaded through bore 2 and the axially outer ends of wire 3 are conductively connected to copper layers in. This is preferably achieved by means of a soft solder which does not tend to corrode the silver wire 3. Low tin content solder, e.g. a solder made up of lead and but 5% tin, serves the intended purpose Well. Reference numeral 4 has been applied to indicate a pair of relatively thick wire leads each having one end conductively connected to one of the outer layers In of unit 1. Each wire lead 4 is provided with a flattened end 4a engaging, and spot-welded to, the outer surface of one of layers la. An encapsulating body 5 of a thermoplastic substance extends all around supporting body 1 and exposes the end of each of the pair of wire leads 4 remote from supporting body 1. To be more specific, encapsulating body 5 covers all surfaces and edges supporting body 1 and defines a continuous outer surface encompassing a solid angle of 360 degrees. The thermoplastic substance of which body 5 is made also fills bore 2.

The current path through the above described device is as follows (FIG. 1): right lead 4, right metal layer 1a, silver wire 3, left metal layer 10, left lead 4.

On occurrence of a current of suflicient intensity and sufiicient duration to cause melting of wire 3 an electric arc will be kindled inside of bore 2 approximately in the median plane of the device. The thermoplastic encapsulating substance, e.g. an epoxy resin, immediately adjacent the point of arc initiation will fuse and bound a bubble within which the arc burns. Vapor streaming from the boiling bubble walls into the inside of the bubble tends to cool and de-ionize the arc path. The diameter of the arc bubble reaches rapidly that of bore 2. This limits the growth of the arc bubble in radial direction. Henceforth the arc-bubble grows but in a direction longitudinally of bore 2. The walls of the channel or bore 2 inside gas-evolving insulating layer 1b give off arcquenching gases when subjected to the direct heat of the arc.

The structure as illustrated in FIGS. 1 and 2 is subject to certain voltage limitations. If the circuit voltage of the circuit into which the device is to be inserted exceeds these limitations, the particular limitations can readily be raised by providing the supporting body 1 with a plurality of transverse bores and by establishing a series connection of fusible wires threaded through these bores.

Referring now to FIGS. 3 and 4, numeral 1' indicates generally an integral unit of copper-clad organic insulating material including outer layers 1a of copper sandwiching inner layer 1b of gas-evolving organic insulating material. Unit 1 has a substantially V-shaped incision 2 including a relatively wide entrance portion 2a and a relatively narrow end portion 212'. A relatively thin fusible silver wire 3 having a center portion arranged in narrow end portion 212 at right angles to the general plane defined by unit 1 conductively interconnects the two metal layers 1a. The ends of wire 3 overlap outer layers 1a and are conductively connected to the same. This is best achieved by means of a non-corrosive soft solder such as the tin-poor solder referred-to above. Leads 4 are relatively thick and each of them has one end 4a which is flattened and spot Welded to one of metal layers 1a. The encapsulating body 5 of a thermoplastic substance extends all around unit 1' and exposes the ends of lead wires 4' remote from unit 1.

It will be apparent that the structure of FIGS. 3 and 4 differs from that of FIGS. 1 and 2 inasmuch as the latter does not require the relatively critical threading operation of its fusible silver wire. It is quite easy to place the latter into the closed end 222 of V-notch or incision 2'.

Upon melting of wire 3 and kindling of an arc an arc bubble is formed enclosing the arc. This bubble is supplied with relatively cool unionized gas which evolves from the narrow end 2b of the V-notch 2b in the gasevolving layer 1!). The arc-bubble remains narrow at the point of arc-inception and widens toward the wide end 2a of the incision or V-notch 2. The structure of FIGS. 3 and 4 is a cross-blast device, i.e., One wherein the arcextinguishing blast of gas is at right angles to the direction of the arc. The latter is largely, or substantially, confined to the narrow end 2b of V-notch 2 and thus cannot escape from the action of the gas evolved from the adjacent walls of V-notch 2'.

The inner layer 1b of the structure of FIGS. 1 and 2 and the inner layer lb of the structure of FIGS. 3 and 4 consist preferably of an organic insulating material embedding a silicious filler having a relatively high heat of fusion. The organic insulating material may be a melamine resin and the filler may be glass-cloth. Such a filler has a high heat absorbing capacity, comparable to that of a quartz sand filler is used in electric fuses for submersing therein fusible conductors. Whereas such sand fillers tend to form fulgurites when subjected to the heat of electric arcs, and whereas fulgurites are good conductors of electricity as long as they are at elevated temperatures, organic material including silicious fillers having a relatively high heat of fusion combine the property of high heat absorption with that of substantial retention of their insulating character at relatively high temperatures.

It will be apparent from the foregoing that we have provided electric fuses that lend themselves particularly well for application in electronic circuitry, including so-called printed circuits, or similar circuits, in which fuses glass enclosures as well as pulverulent arc-quenching fillers have been dispensed with, and in which fuses both the encapsulation and the support for the fuse wire or fusible element are adapted to operate as arc-quenching means. It will further be apparent that we have provided electric fuses wherein the action of an arc-bubble resulting from fusion of an encapsulating body, the action of a source of unionized gas formed by a spacer layer arranged between two metal layers, and the heat absorbing action of a silicious substance having a relatively high heat of fusion combine to achieve arc extinction in a relatively limited space without outward release of unionized or incandescent gases.

It will also be apparent that fuses according to this invention are substantially in the shape of a solid of revolution or ellipsoid, having an axis which is at right angles to the plane defined by layers la, 111, la which is considerably shorter than the axis situated in the general plane defined by layers la, 1b, 1a.

Although but two embodiments of the invention have been illustrated and described it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

it. An electric fuse comprising:

a supporting body formed by an integral laminate having a pair of parallel outer layers of metal sandwiching an inner layer of an organic insulating material evolving gas when subjected to the heat of an electric arc;

a relatively thin fusible wire extending over an edge region of said supporting body and conductively interconnecting said pair of outer layers thereof;

a pair of relatively thick leads having spaced parallel ends sandwiching said pair of outer layers therebetween, each of said pair of leads being conductively connected to one of said pair of outer layers of said supporting body; and

an encapsulating body extending all around said supporting body covering all surfaces and edges thereof and exposing the end of each of said pair of leads remote from said supporting body, said encapsulating body having a smaller width at right angles to said pair of outer layers than in the general plane of said outer layers.

2. An electric fuse comprising:

a supporting body formed of an integral laminate having a pair of parallel outer layers of metal sandwiching an inner layer of an organic insulating material embedding a silicious filler having a relatively high heat of fusion;

a relatively thin fusible wire extending over an edge region of said supporting body and conductively interconnecting said outer layers thereof;

a pair of relatively thick leads having spaced parallel ends sandwiching said pair of outer layers therebetween, each of said pair of leads being conductively connected to one of said pair of outer layers of said supporting body; and

an encapsulating body extending all around said supporting body, covering all surfaces and edges thereof and exposing the end of each of said pair of leads remote from said supporting body, said encapsulating body having a smaller width at right angles to said pair of outer layers than in the general plane of said outer layers.

3. An electric fuse as specified in claim 2 wherein said organic insulating material is an epoxy resin and wherein said filler is fiber-glass-cloth.

4. An electric fuse comprising:

an integral unit of copper-clad organic insulating material including an inner layer of said insulating material sandwiched between a pair of outer layers of copper, said unit having a fine bore extending transversely through said inner layer and-said outer layers;

a relatively thin fusible wire threaded through said bore and conductively interconnecting said outer layers of said unit;

a pair of relatively thick lead wires having spaced parallel ends sandwiching said pair of outer layers therebetween, each of said pair of leads being spot-welded at juxtaposed points to one of said pair of outer layers of said unit; and

an encapsulating body of a thermoplastic substance extending all around said unit and covering all surfaces and edges thereof to define a continuous outer surface encompassing a solid angle of 360 degrees, said encapsulating body exposing the ends of each of said pair of lead wires remote from said unit, said encapsulating body having a smaller width at right angles to said pair of outer layers than in the general plane of said outer layers.

5. An electric fuse comprising:

an integral unit of copper-clad organic insulating material including an inner layer of said insulating ma terial sandwiched between a pair of outer layers of copper, said unit having a substantially V-shaped incision including a relatively Wide open entrance portion and a relatively narrow closed end portion;

a relatively thin fusible wire having a center portion arranged in said end portion at right angles to the plane defined by said unit and having ends overlapping said pair of outer layers and conductively connected to said outer layers;

a pair of relatively thick leads having spaced parallel ends sandwiching said pair of outer layers therebetween, each of said pair of leads being conductively connected to one of said pair of outer layers of said unit; and

an encapsulating body of a thermoplastic substance extending all around said unit and covering all surfaces and edges thereof to define a continuous surface encompassing a solid angle of 360 degrees, said encapsulating body exposing the ends of each of said pair of lead Wires remote from said unit, said en- 5 6 capsulating body having a smaller width at right 2,392,703 1/ 1946 Schurig 200-420 angles to said pair of outer layers than in the general 2,667,549 1/ 1954 Fahnoe et a1 200120 plane of said outer layers. 2,830,156 4/1958 Burgess 200131 2,856,488 10/ 195 8 Kozacka 200120 References Cited y h Ex m n r 5 2,895,031 7/1959 Kozacka 200 120 UNITED STATES PATENTS BERNA DA. ILHEAN ,P- E 2,328,825 9/1943 McMahon 200-420 R G Y 

1. AN ELECTRIC FUSE COMPRISING: A SUPPORTING BODY FORMED BY AN INTEGAL LAMINATE HAVING A PAIR OF PARALLEL OUTER LAYERS OF METAL SANDWICHING AN INNER LAYER OF AN ORGANCI INSULATING MATERIAL EVOLVING GAS WHEN SUBJECTED TO THE HEAT OF AN ELECTRIC ARC; A RELATIVE THIN FUSIBLE WIRE EXTENDING OVER AN EDGE REGION OF SAID SUPPORTING BODY AND CONDUCTIVELY INTERCONNECTING SAID PAIR OF OUTER LAYERS THEREOF; A PAIR OF RELATIVELY THICK LEADS HAVING SPACED PARALLEL ENDS SANDWICHING SAID PAIR OF OUTER LAYERS THEREBETWEEN, EACH OF SAID PAIR OF LEADS BEING CONDUCTIVELY CONNECTED TO ONE OF SAID PAIR OF OUTER LAYERS OF SAID SUPPORTING BODY; AND AN ENCAPSULATING BODY EXTENDING AL AROUND SAID SUPPORTING BODY COVERING ALL SURFACES AND EDGES THEREOF AND EXPOSING THE END OF EACH OF SAID PAIR OF LEADS REMOTE FROM SAID SUPPORTING BODY, SAID ENCAPSULATING BODY HAVING A SMALLER WIDTH AT RIGHT ANGLES TO SAID PAIR OF OUTER LAYERS THAN IN THE GENERAL PLANE OF SAID OUTER LAYERS. 